1. Field of the Invention
The invention relates to semiconductor device fabrication on silicon wafers or other substrates and, more particularly, to solvent compositions and methods for their use in removing excess amounts of stress buffer coating materials from the coated or top side periphery of the wafer, from the wafer edge, and from the non-coated or bottom side of the wafer.
2. Description of the Prior Art
Stress buffer coating materials such as polyimide and polybenzoxazole films have proven useful for improving the reliability of packaged semiconductor devices (P. V. Robock and L. T. Nguyen, in R. Tummala and E. J. Rymaszewski, ed., Microelectronics Packaging Handbook, Van Nostrand Reinhold, New York, 540, (1989)). Stress buffer coatings are formed on semiconductor devices in layers 2-25 microns thick using standard semiconductor industry methods using the consecutive process steps of spin coating a stress buffer coating composition such as those based on polyirmide (K. Horie and T. Yamashita, ed., Photosensitive Polyimides: Fundamentals and Applications, Technomic Publishing Co., Inc., Lancaster, Pa., (1995) and R. Hopla, P. Falcigno, S. Hagen, J. Herbert, D. Huglin, H. J. Kirner, T. Maw, A. Schaffner, W. Weber, and O. Rhode, Proceedings of The 10th International SPE Conference on Photopolymers, Mid-Hudson SPE, 1994, pp. 463-469) or polybenzoxazole (“A Novel Positive Working Photosensitive Polymer For Semiconductor Surface Coating”, H. Makabe, T. Banba, and T. Hirano, Journal of Photopolymer Science and Technology, Volume 10, Number 2(1997) pp. 307-312) precursor polymers, edge bead removal, soft baking the wet coating to yield a rigid film that does not flow at room temperature, forming relief structures in the film using a lithographic process, followed by final curing in an oven or furnace at a temperature and time sufficient to convert the precursor into a polyimide or polybenzoxazole stress buffer film. In the case of photosensitive stress buffer compositions, the relief structures may be formed by direct lithographic exposure. In the case of non-photosensitive stress buffer compositions, an indirect lithographic process using image transfer through a sacrificial photoresist film which is applied to the stress buffer coating precursor film in a separate and subsequent coating step may be used. For either the direct or indirect lithographic process, an image is formed in the photosensitive film by exposing the film to actinic radiation that has first been passed through a patterning mask. Actinic radiation in the wavelength range of 240 to 500 nm can be used and the emission lines at 365, 405, and 436 nm of the mercury lamp are particularly useful for this purpose.
When spin coated, stress buffer coating formulations leave a band or “edge bead” of material at the wafer periphery that is significantly thicker than the coating on the rest of the wafer area. This region of significantly greater coating thickness varies from 1-3 millimeters in width and is found around the complete circumference of the wafer. The thickness of this edge bead can be twice that of the remainder of the coated area. Additionally, coating material is deposited on the edge of the wafer and very frequently flows over the wafer edge and deposits on the non-coated or bottom side of the wafer thereby contaminating the bottom and edge of the wafer.
The presence of the edge bead causes difficulties in maintaining optimal optical focus when step and repeat exposure tools, i.e., steppers, are used to image the photosensitive coatings resulting in a loss in pattern resolution. Specifically, an image that is in focus in the interior region of the wafer where no edge bead is present is out of focus in the edge bead containing region resulting in poor pattern definition and a consequent loss in manufacturing yield. Because the thick edge bead has a greater optical absorbance than the remainder of the film, larger exposure doses are required to form a latent image in the edge bead. Attempts to image the edge bead result in an overexposure condition in the remainder of the film resulting in poor pattern definition. Excess material on the wafer edge is prone to chipping and thus produces particulate contamination that can adversely affect the yield of processes. Excess material on the bottom side of wafers can result in contamination of semiconductor device manufacturing equipment. Step and repeat exposure tools are especially affected because the bottom side contamination prevents the wafer from being positioned orthogonally to the mask image. If the contamination is transferred to the stepper wafer stage, the focus of subsequent wafers may be compromised. This transferred contamination leads to an out-of-focus condition on some regions of the wafers that are subsequently processed in the exposure equipment. Additionally, the desired functions of the wafer handling devices on the coating equipment and on the hot plates that are used to bake the coated substrate may be adversely affected by the presence of contamination on the wafer edge and bottom side. Accordingly, it is very important to mitigate the adverse effects of the stress buffer coating material edge bead and the edge and bottom side contamination prior to the exposure step by using an edge bead remover (EBR) solvent to reduce the thickness of the edge bead and to remove the edge and bottom side contamination.
Edge bead removal solvents have been developed for removing the edge bead and bottom side contamination resulting from spin coated photoresist compositions that are primarily composed of novolac resin binders and photoactive compounds that are ortho-quinonediazide derivatives. FIG. 2 of U.S. Pat. No. 4,518,678 provides a cross sectional diagram, reproduced herein as FIG. 1, showing a photoresist coating based on a composition containing a novolac resin and an ortho-quinonediazide on a wafer substrate prior to the application of an edge bead removal solvent. The essential features of FIG. 1 are the substrate 1 and the coating 2 wherein 2b represents the coating that extends around the edge of the substrate and 2c represents the coating that extends onto the edge of the substrate bottom surface. FIG. 3 of U.S. Pat. No. 4,518,678, reproduced herein as FIG. 2, illustrates the result of performing an edge bead removal process on the photoresist coating of FIG. 1. The essential features of FIG. 2 are the peripheral band of exposed substrate 1a on the upper surface of the substrate 1, the exposed substrate edge 1b, and the peripheral bottom edge of the substrate 1c from which bottom side resist contamination has been removed.
Several solvents and solvent mixtures have been described and employed as edge bead removers for photoresists. For example, U.S. Pat. No. 4,886,728 teaches the application of a mixture of ethyl lactate and methyl ethyl ketone as an EBR solvent for removal of photoresist from the edge or bottom side of wafers. U.S. Pat. No. 5,426,017 describes the use of mixtures containing C4 to C8 alkyl acetates, C4 to C8 alkyl alcohols, and water as EBR compositions for use with photoresists based on novolac resin binders and ortho-quinonediazide photoactive compounds. U.S. Pat. No. 5,814,433 teaches the use of a mixture of ethyl lactate and N-methylpyrollidone (NMP) as an EBR composition for removal of photoresist. Since it has been established that NMP has detrimental effects on the performance of chemically amplified 248 and 193 nm photoresists (U.S. Pat. No. 6,277,546 B1; “Influence of Polymer Properties On Airborne Chemical Contamination of Chemically Amplified Resists”, W. D, Hinsberg, S. A. MacDonald, N. J. Clecak, C. D. Snyder, and H. Ito, SPIE vol. 1925, pp. 43-52, 1993), use of NMP-containing compositions is prohibited in many semiconductor fabrication facilities where such chemically amplified resists are used. U.S. Pat. No. 5,362,608 describes the use of tetrahydrofurfuryl alcohol as the active component of NMP-free EBR compositions stated to be compatible with 248 nm photoresist. U.S. Pat. No. 5,866,305 describes the application of an EBR and bottom side cleaner composition that includes at least ethyl lactate and ethyl 3-ethoxypropionate, and preferentially includes less than 10% gamma-butyrolactone. U.S. Pat. No. 6,015,467 describes an edge bead removing process using a solvent composition that includes one of dipropylene glycol monoalkyl ether alone, a mixture of this ether and an easily volatile organic solvent, and a mixture of the ether, a volatile organic solvent, and an alkaline aqueous solution. U.S. Pat. No. 6,117,623 describes an EBR composition containing a lactone compound together with a second solvent selected from the group consisting of an alkoxybenzene or an aromatic alcohol with the preferred embodiment stated to be mixtures of gamma-butyrolactone and anisole. U.S. Pat. No. 6,159,646 teaches the use of compositions containing ethyl lactate, ethyl 3-ethoxypropionate and gamma-butyrolactone or a mixture of ethyl lactate and ethyl 3-ethoxypropionate or a mixture of ethyl lactate and gamma-butyrolactone as preferred compositions for use in a method for reworking photoresist coated wafers by dissolving the photoresist coating from the entire area of the wafer. JP 10-097079 teaches the use of an edge bead remover for photoresists comprising 60-90% ethyl lactate and 10-40% gamma-butyrolactone. JP 06-212193 claims solvent compositions containing an alkyl lactate which could be methyl, ethyl, isopropyl, or butyl lactates and further containing at least one of NMP, DMF, DMAc, methyl acetate, ethyl acetate, GBL, DMSO, or sulfolane with the composition being directed to removing residue remaining after resist stripping. GB 2357343 claims a photoresist stripper composition comprising a mixture of acetone, GBL, and an ester solvent and a method for use.
JP 2936897 claims an EBR solvent composition for use with polyimide precursors containing 70-85% N,N,-dimethylformamide and 15-30% methanol. JP 2000-31969 claims a solvent composition containing 60-100% of gamma-butyrolactone and preferably 1-40% of a solvent with a boiling point range lower than gamma-butyrolactone directed to edge bead removal of heat resistant polymer having imide rings or other ring structures. The cited examples of heat resistant polymers include polyimide, polyamideimide, polybenzoxazole, polybenzthiazole, and polybenzimidazole, and the polymer may be a copolymer or a blend.
Of importance and consequence is the fact that the edge bead removal process requirements for stress buffer coatings are different from the edge bead process requirements for processing standard photoresists, as described above in the prior art, in several important ways. First, the solubility of the components of stress buffer coating compositions is different than the solubility of the components of photoresists. This solubility difference is particularly true relative to the polymer components of stress buffer coating compositions compared to the polymer components of photoresists. The polymers used in photoresist compositions include cresol-formaldehyde novolacs, derivatized polyhydroxystyrenes, and derivatized polyacrylate copolymers while stress buffer coating compositions are based on polyarylamide derivatives and polyarylamide-esters. Solvents used in photoresist compositions are typically esters and include ethyl lactate, propylene glycol monomethyl ether acetate, and other solvents and solvent mixtures with a boiling point range of 120-170° C.
In general, photoresist solvents alone are not good solvents for stress buffer coating compositions. Solvents used in stress buffer coating compositions are typically polar, aprotic solvents with a boiling point range of 190-230° C. and are exemplified by N-methylpyrollidone (NMP, boiling point 202° C.) and gamma-butyrolactone (GBL, boiling point 204-205° C.). Because EBR solvents formulated for use in photoresist EBR processes are based on photoresist solvents, it is not surprising that difficulties occur when stress buffer coating EBR processes are attempted using photoresist EBR compositions.
Second, stress buffer coating processing involves forming coatings that range in thickness from 4-50 microns after soft baking while photoresists are commonly processed in a film thickness range of 0.5 to 3 microns after baking. Consequently, the material mass of the stress buffer coating edge bead is substantially greater than the material mass of a photoresist edge bead.
Third, the rheology of the spin coated films is very different. Photoresists form firm, non-flowing films during the spin coating process through solvent evaporation while stress buffer coating compositions form wet or soft, gel-like coatings that can flow after spin coating. This rheological difference between photoresist and stress buffer coatings is due to both the film thickness difference and to the difference in the volatility of the casting solvents. Since EBR processing follows the spin coating process, the wet stress buffer coatings continue to move during any of the substrate spinning operations that occur during the edge bead removal process and this movement complicates the EBR process. Indeed, after EBR solvent application has been completed, a shorter, lower spin speed process cycle is frequently necessary to reduce the tendency of the dissolved stress buffer coating material to move over the wafer edge and onto the bottom side of the wafer.
EBR processing of stress buffer coatings present unique problems compared to EBR processing of photoresists. In the case of photoresists, the front edge bead can be removed leaving a clear clean outer edge of exposed wafer about 1-3 mm wide as illustrated in FIG. 2. Because the wet stress buffer coating flows during wafer spinning, the front side edge bead of stress buffer coating cannot be completely removed during the edge bead removal process. FIG. 3 is a cross-sectional side view of a wafer that has been spin coated with stress buffer coating composition and softbaked. The essential features of FIG. 3 are the wafer 1 and the stress buffer coating 3 wherein 3a represents the thick edge bead characteristic of stress buffer coatings, 3b represents the coating that extends around the edge of the substrate, and 3c represents the coating that extends onto the edge of the substrate bottom surface. The goal of EBR processing on stress buffer coating films is to minimize the negative impact of the edge bead on wafer processing and yield by reducing the thickness of the edge bead region (3a) to a thickness as nearly equal to the thickness of the rest of the film as possible while removing contamination from the bottom surface of the substrate.
If an edge bead solvent containing dissolved stress buffer coating material remains on the wafer for an extended period of time, it can flow or back-stream to the bottom side of the wafer resulting in contamination of the bottom surface as illustrated in 3c of FIG. 3. Use of a volatile EBR composition provides a solvent mixture that dries more quickly and thus prevents back streaming of the dissolved material and minimizes contamination of areas that were cleaned during the edge bead removal process. However, fast drying is not the only requirement. The solvent or solvent mixture must be strong enough to reduce the thickness of the edge bead and remove any small spots of coating material that has dried on the backside side of the wafer. These small spots of material on the backside of the wafer are formed by spattering of coating material during spin coating. These small spots of material are not as soluble as the top side wet film due to convective drying during wafer spinning.
Another important reason for having good solvency is the fact that the spin coating and edge bead removal processes are performed in the same coating cup. The use of solvents or solvent mixtures that are poor solvents can lead to a build-up of residues in the coating equipment spinning cup. These residues can add another source of contamination as the residue can be deposited onto the wafer by splashing off the walls of the coater cup. Additionally, the residue in the coating cup can act to prevent or impede waste spun off the wafer and into the coater cup from flowing into waste containers.
In summary, it is important that a stress buffer coating EBR composition and process meet the combined requirements of: (1) reducing the thickness of the coating edge bead, (2) having drying characteristics such that dissolved material is not deposited onto the bottom of the wafer, (3) removing partially dried coating material from the bottom side of the wafer, and (4) not having a tendency to promote the accumulation of coating waste in the coating cup.
Therefore, the present invention addresses the need to improve the edge bead removal process for stress buffer coatings due to the inadequate performance of photoresist EBR solvents by providing a novel EBR composition that meets the above-requirements.